For over sixty years, light valves have been proposed for use for modulation of light. As used herein, a light valve comprises a cell formed of two walls that are spaced apart by a small distance, at least one wall being transparent, the walls having electrodes thereon usually in the form of transparent conductive coatings. The cell contains an activatable light-modulating material which may be either a liquid suspension or a plastic film in which droplets of a liquid suspension are distributed and encapsulated. The liquid suspension (sometimes herein called a liquid light valve suspension) comprises small particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension exhibit random Brownian movement, and hence a beam of light passing into the cell is reflected, transmitted or absorbed, depending upon the cell structure, the nature and concentration of the particles and the energy content of the light. When an electric field is applied through the light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. Light valves have been proposed for many purposes including, e.g., alpha-numeric displays, television displays, windows, sunroofs, sunvisors, mirrors, eyeglasses and the like to control the amount of light passing therethrough. Light valves based upon the use of a suspension of particles to modulate light are known as "suspended particle devices" or "SPDs".
For many applications, it is preferable for the activatable material of an SPD light valve to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window, a plastic film in which droplets of liquid suspension are distributed is preferable to a liquid suspension alone because hydrostatic pressure effects e.g., bulging, associated with a high column of liquid suspension, can be avoided through use of a film, and the risk of possible leakage can also be avoided. Also, in a plastic film, the particles are present only within very small droplets, and hence do not noticeably agglomerate when the film is repeatedly activated with a voltage.
One type of light valve film for an SPD light valve which uses microcapsules of suspended particles dispersed in a solid matrix layer is disclosed in U.S. Pat. No. 4,919,521. A second type of SPD light valve film made by phase separation from a homogeneous solution is disclosed in U.S. Pat. No. 5,409,734. SPD light valve films made by crosslinking a cross-linkable film-forming material with a chemical cross-linking agent are disclosed in U.S. Pat. Nos. 5,463,491 and 5,463,492 assigned to the assignee of the present invention. All of those patents and other patents cited herein are incorporated herein by reference.
There are several advantages of UV-curing an SPD film instead of using heat to cure it as is disclosed in U.S. Pat. Nos. 5,463,491 and 5,463,492. An SPD film cured with heat begins to cure as soon as catalyst is added, whereas a UV-curable film will only cure when exposed to ultraviolet radiation. UV-curing also avoids prolonged exposure of the film to heat which might damage the film. Finally, UV-curing can be accomplished much quicker than heat curing. In air, UV-curing can often be effected in less than 1 minute, and in an oxygen-free atmosphere, in only a few seconds.